Imaging devices such as fax machines capture images by scanning the image at a known, constant rate. For example, in a typical fax machine, a stepper motor draws the sheet bearing the image through a slot to move the image past a fixed scanning head at a constant rate. At any particular time, the scanning head "sees" only a narrow, linear portion of the image adjacent the scanning head. Because the scan rate is constant and predetermined, the fax machine's control circuity can easily be activated at fixed, sequential time intervals, to transfer the image portion seen during each interval into a buffer. The buffer accumulates the image portions to yield a full digital representation of the scanned image. Instead of moving the image past a fixed scanning head as described above, the image may remain in a fixed position while the stepper motor moves the scanning head past the image at a constant rate, as in a typical flat bed scanner.
Prior art devices like those mentioned above use relatively expensive, bulky stepper motors to maintain the scan rate constant to a high degree of precision. Some prior art devices, such as hand-held scanners, use less bulky encoders coupled to rollers to synchronize the scanning process as the scanner is hand-manoeuvred, at variable speed, over the object being imaged. Although the scan rate varies, it can be determined by electrical and/or mechanical coupling between the roller(s) and encoder(s). However, such arrangements require expensive high precision encoders and/or considerable mechanical complexity to achieve high resolution in the scanned image.
The present invention provides a method and apparatus for capturing an image of a moving object using one or more linear arrays, without the need for high precision stepper motors, and without the need for mechanically measuring the speed of the object.